Semiconductor device and method

ABSTRACT

A transistor based on topological insulators is provided. In an embodiment a topological insulator is used to form both the channel as well as the source/drain regions, wherein the channel has a first thickness such that the topological insulator material has properties of a semiconductor material and the source/drain regions have a second thickness such that the topological insulator has properties of a conductive material.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.16/848,356, filed on Apr. 14, 2020, entitled “Semiconductor Device andMethod,” which is a division of U.S. patent application Ser. No.16/050,800, filed on Jul. 31, 2018, entitled “Semiconductor Device andMethod,” now U.S. Pat. No. 10,636,651 issued on Apr. 28, 2020, which isa division of U.S. patent application Ser. No. 15/151,100, filed on May10, 2016, entitled “Semiconductor Device and Method,” now U.S. Pat. No.10,109,477 issued on Oct. 23, 2018, which application claims priority toand the benefit of U.S. Provisional Application No. 62/273,628, filed onDec. 31, 2015, entitled “Tunable Gap Ultra-Thin-Body Transistor Based ona Topological Insulator,” which applications are hereby incorporatedherein by reference in their entirety.

BACKGROUND

Semiconductor devices are used in a variety of electronic applications,such as personal computers, cell phones, digital cameras, and otherelectronic equipment, as examples. Semiconductor devices are typicallyfabricated by sequentially depositing insulating or dielectric layers,conductive layers, and semiconductive layers of material over asemiconductor substrate, and patterning the various material layersusing lithography to form circuit components and elements thereon.

Transistors are circuit components or elements that are often formed onsemiconductor devices. Many transistors may be formed on a semiconductordevice in addition to capacitors, inductors, resistors, diodes,conductive lines, or other elements, depending on the circuit design.Improvements in transistor designs are desired.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a formation of a channel layer, a gate dielectriclayer, and a gate electrode layer in accordance with some embodiments.

FIGS. 2A-2C illustrate band gaps of topological insulators in accordancewith some embodiments.

FIG. 3 illustrates a patterning of the channel layer, the gatedielectric layer, and the gate electrode layer in accordance with someembodiments.

FIGS. 4A-4B illustrate a formation of source/drain regions in accordancewith some embodiments.

FIGS. 5A-5C illustrate a formation of the channel and source/drainregions in accordance with some embodiments.

FIGS. 6A-6B illustrate a formation of the channel on the gate dielectricin accordance with some embodiments.

FIG. 7 illustrates a formation of a second channel layer, a second gatedielectric layer, and a second gate electrode in accordance with someembodiments.

FIGS. 8A-8B illustrate a formation of source/drain regions in accordancewith some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Embodiments will now be described with respect to a tunable gapultra-thin body transistor which is based on a topological insulator.However, the embodiments described herein may be applied in any suitableapplication.

With reference now to FIG. 1 , there is illustrated a substrate 101, afirst channel layer 103, a first gate dielectric layer 105, and a firstgate electrode layer 107. In an embodiment the substrate 101 maycomprise bulk silicon, doped or undoped, or an active layer of asilicon-on-insulator (SOI) substrate. Generally, an SOI substratecomprises a layer of a semiconductor material such as silicon,germanium, silicon germanium, SOI, silicon germanium on insulator(SGOI), or combinations thereof. Other substrates that may be usedinclude sapphire, multi-layered substrates, gradient substrates, orhybrid orientation substrates. Any suitable substrate may be utilized.

The first channel layer 103 may be formed over the substrate 101 andwill be used to form a first channel 305 (not illustrated in FIG. 1 butillustrated and discussed below with respect to FIG. 3 ) for asingle-gate transistor 400 (also not illustrated in FIG. 1 as beingcompleted but illustrated and described below with respect to FIG. 4A).In an embodiment the first channel layer 103 may be a topologicalinsulator material wherein the material has a bulk structure with aninsulating or semiconducting (gapped) structure as well as conducting(gapless) edges or surfaces due to non-trivial topology of a bandstructure caused by interactions between spin and orbital degrees offreedom. In particular embodiments in which the first channel layer 103is a topological insulating material, the first channel layer 103 may bea material such as Bi₂Se₃, Bi₂Te₃, Sb₂Te₃, or tetradymite-like ternarycompounds with a structure such as M₂X₂Y such as Bi₂Te₂Se, Bi₂Te₂S,Bi₂Se₂S, Sb₂Te₂Se, Sb₂Te₂S, or the like. However, any suitabletopological insulator may be utilized.

Additionally, with respect to the materials utilized for the firstchannel layer 103, the material of the first channel layer 103 will havea critical thickness T_(c), wherein the thickness of the material of thefirst channel layer 103 will determine the properties of the material ofthe first channel layer 103 and the properties of the material for thefirst channel layer 103 will change as the thickness of the material forthe first channel layer 103 changes. For example, in a particularembodiment in which Bi₂Se₃ is utilized as the material for the firstchannel layer 103, the Bi₂Se₃ will have a critical thickness of sixquintuple layers (e.g., layers of Se—Bi—Se—Bi—Se), below which theBi₂Se₃ will have properties of a semiconductor material and above whichthe Bi₂Se₃ will have properties of a topological insulator which hasbulk insulator properties along with conductive surface states.

FIG. 2A helps to illustrate this change and separation of propertiesbased upon thicknesses. When the thickness of a Bi₂Se₃ film is reducedto several nanometres, the surface-state wavefunctions from the twosurfaces of Bi₂Se₃ film are interfered and overlapped. Therefore, a gapopens and no surface state exists. As illustrated in FIG. 2A, a singlequintuple layer (1QL) of Bi₂Se₃ has an energy band gap that does notallow for the conduction of electricity and causes the Bi₂Se₃ to havethe properties of a semiconductor material. However, as illustrated inFIG. 2B, as the number of layers of Bi₂Se₃ increases, the surface statescan very well exist when the critical thickness of Bi₂Se₃ has beenreached (which for Bi₂Se₃ is about 6 quintuple layers), the band gap isbridged, and the Bi₂Se₃ will have the properties of a topologicalinsulator, such that the material is an insulator but which hasconductive surface states such that electricity will flow along thesurface of the topological insulator. As such, the thickness of thematerial utilized for the first channel layer 103 determines itsproperties, and control of this thickness will also control theproperties that are obtained from the formation of the first channellayer 103.

FIG. 2C helps to illustrate this change in properties based upon thethickness of the material of the first channel layer 103 in anotherfashion. In particular, FIG. 2C illustrates the bandgap of each numberof quintuple layers from 1 quintuple layer of Bi₂Se₃ to 7 quintuplelayers of Bi₂Se₃ and illustrates that at 6 quintuple layers the bandgapreaches zero and the Bi₂Se₃ has the properties of a topologicalinsulator with metallic surface states that allows for the flow ofelectricity. However, below 6 quintuple layers, there is a non-zerobandgap which causes the Bi₂Se₃ to have the properties of asemiconductor material.

In order to function appropriately as the first channel 305, the firstchannel layer 103 is formed to have a first thickness T₁ that is belowthe critical thickness T_(c) (shown in relative located in FIG. 1 ) ofthe material used for the first channel layer 103. In a particularembodiment in which the first channel layer 103 is Bi₂Se₃, Bi₂Te₃, orSb₂Te₃, the first channel layer 103 is formed to have a thickness ofless than 6 quintuple layers, such as having a thickness of 1 quintuplelayer, or about 1 nm. However, any suitable thickness may be utilizeddepending upon the properties of the topological insulator materialbeing utilized. By forming the first channel layer 103 to have the firstthickness T₁ below the critical thickness T_(c) of the material chosenfor the first channel layer 103, the first channel layer 103 will have amaterial with semiconductor properties and not conductive properties.

The first channel layer 103 may be formed using a process such as anepitaxial growth process. In a particular embodiment in which the firstchannel layer 103 is formed from a material such as Bi₂Se₃, theepitaxial growth process may proceed at a temperature of between about100° C. and about 500° C., and at a pressure less than about 2.0×10⁻⁹Torr, using any suitable source or sources for bismuth and selenium,such as evaporated high-purity Bi (99.99%) and Se (99.99%). However, anysuitable growth or deposition process, such as an atomic layerdeposition process or the like, may also be used.

Additionally, the epitaxial growth process may be continued for a timethat is sufficient to grow the first channel layer 103 to the firstthickness T₁ without growing the first channel layer 103 to a thicknessgreater than the critical thickness T_(c). In an embodiment in which thedeposition rate of Bi₂Se₃ films is about 0.67 angstrom/min, theepitaxial growth process may be performed for a first time of betweenabout 70 sec and about 270 sec. However, any suitable time may beutilized.

Once the first channel layer 103 has been formed, a first gatedielectric layer 105 and a first gate electrode layer 107 may be formedover the first channel layer 103. The first gate dielectric layer 105may be formed from a high permittivity (high-k) material (e.g., with arelative permittivity greater than about 5) such as aluminum oxide(Al₂O₃), hafnium oxide (HfO₂), hafnium oxynitride (HfON), lanthanumoxide (La₂O₃), or zirconium oxide (ZrO₂), or combinations thereof, withan equivalent oxide thickness of about 0.5 nm to about 2 nm.Additionally, any combination of silicon dioxide, silicon oxynitride,and/or high-k materials may also be used for the first gate dielectriclayer 105. The first gate dielectric layer 105 may be formed using aprocess such as atomic layer deposition, chemical vapor deposition,sputtering, or the like.

The first gate electrode layer 107 may comprise a conductive materialand may be selected from a group comprising of gold, titanium, platinum,aluminum, polycrystalline-silicon (poly-Si), poly-crystallinesilicon-germanium (poly-SiGe), metallic nitrides, metallic silicides,metallic oxides, and metals. Examples of metallic nitrides includetungsten nitride, molybdenum nitride, titanium nitride, and tantalumnitride, or their combinations. Examples of metallic silicide includetungsten silicide, titanium silicide, cobalt silicide, nickel silicide,platinum silicide, erbium silicide, or their combinations. Examples ofmetallic oxides include ruthenium oxide, indium tin oxide, or theircombinations. Examples of other metals that may be used includetantalum, tungsten, copper, molybdenum, nickel, etc. Any suitablematerial may be used to form the first gate electrode layer 107.

The first gate electrode layer 107 may be deposited by sputterdeposition, chemical vapor deposition (CVD), or other techniques knownand used in the art for depositing conductive materials. The thicknessof the first gate electrode layer 107 may be in the range of about 200angstroms to about 4,000 angstroms. Dopants may or may not be introducedinto the first gate electrode layer 107 at this point. Dopants may beintroduced, for example, by molecular doping techniques thru chargetransfer.

FIG. 3 illustrates that, once the first gate electrode layer 107 hasbeen formed, the first gate electrode layer 107 may be patterned to forma first gate electrode 301. The first gate electrode 301 may be formedby depositing and patterning a first gate mask (not illustrated in FIG.3 ) on the first gate electrode layer 107 using, for example, depositionand photolithography techniques known in the art. The first gate maskmay incorporate commonly used masking materials, such as (but notlimited to) photoresist material, silicon oxide, silicon oxynitride,and/or silicon nitride. Once the first gate mask has been placed, thefirst gate electrode layer 107 may be etched using plasma etching toform the first gate electrode 301. In an embodiment the first gateelectrode layer 107 may be patterned to have a first width W₁ of betweenabout 7 nm and about 100 μm.

FIG. 3 also illustrates a patterning of the first gate dielectric layer105 and the first channel layer 103. In an embodiment the patterning ofthe first gate dielectric layer 105 and the first channel layer 103 maybe initiated by removing the first gate mask using, e.g., an ashing orother removal process, and a second gate mask may be deposited andpatterned. In an embodiment the second gate mask may be deposited andpatterned using, for example, deposition and photolithography techniquesknown in the art. The second gate mask may incorporate commonly usedmasking materials, such as (but not limited to) photoresist material,silicon oxide, silicon oxynitride, and/or silicon nitride. Once thesecond gate mask has been placed, the first gate dielectric layer 105and the first channel layer 103 may be etched using plasma etching toform the first gate dielectric 303 and the first channel 305. In anembodiment the first gate dielectric 303 and the first channel 305 maybe patterned to have a second width W₂ of between about 7 nm and about100 μm.

FIGS. 4A-4B illustrate a formation of source/drain regions 401 incontact with the first channel 305. In an embodiment the source/drainregions 401 comprises a material with a critical thickness T_(c) asdescribed herein, wherein the properties of the material used for thesource/drain regions 401 are dependent at least in part upon thethickness of the material. In some embodiments the material of thesource/drain regions 401 is a topological insulator and, in particularembodiments, is the same topological insulator material as the firstchannel 305. For example, in an embodiment in which the first channel305 is Bi₂Se₃, the source/drain regions 401 are also Bi₂Se₃, althoughany suitable material may be used.

However, while the first channel 305 is formed using a topologicalinsulator with a thickness that keeps the properties of the topologicalinsulator as a semiconductor material (e.g., below 6 quintuple layersfor Bi₂Se₃, such as 1 quintuple layer), the source/drain regions 401 areformed using a topological insulator material with a second thickness T₂that is greater than the critical thickness T_(c) of the material. Byforming the material of the source/drain regions 401 to have the secondthickness T₂ greater than the critical thickness T_(c), the material ofthe source/drain regions 401 will have different properties than thematerial of the first channel 205, such as by having the properties of atopological insulator with bulk insulating properties as well as havingsurfaces with metallic behavior that allows for the conduction ofelectricity along the surface. In particular embodiments in whichBi₂Se₃, Bi₂Te₃, or Sb₂Te₃ are utilized for the source/drain regions 401,the source/drain regions 401 may be formed with the second thickness T₂of six quintuple layers or greater, such as about 6 nm or greater. Thesource/drain regions 401 may be formed to have a third width W₃ ofbetween about 7 nm and about 100 μm.

In an embodiment the source/drain regions 401 may be formed from similarmaterials and using similar processes as the first channel layer 103(described above with respect to FIG. 1 ). For example, the source/drainregions 401 may be formed using a process such as molecular beamepitaxial growth. However, for the process of forming the source/drainregions 401, instead of continuing the process for the first time ofbetween about 70 sec and about 270 sec, which would result in the firstthickness T₁ below the critical thickness T_(c), the epitaxial growthprocess for the source/drain regions 401 is continued for a second timeof between about 6 min and about 120 min, such that the source/drainregions 401 have the second thickness T₂ greater than the criticalthickness T_(c). However, any suitable process may be utilized to formthe source/drain regions 401.

FIG. 4B illustrates a top-down view of the single-gate transistor 400.As can be seen, the first gate dielectric 303 may be formed to have afirst length L₁ of between about 7 nm and about 100 μm. Additionally,the source/drain regions 401 may be formed to have a second length L₂ ofbetween about 7 nm and about 100 μm. The first gate electrode 301 may beformed to have a third length L₃ of between about 7 nm and about 100 μm.However, any suitable dimensions may be utilized.

By forming the first channel 305 to have the first thickness T₁ suchthat the first channel 305 has the properties of a semiconductormaterial and by also forming the source/drain regions 401 to have thesecond thickness T₂ such that the source/drain regions 401 have theproperties of a topological insulator with a conductive surface, thesame material (e.g., Bi₂Se₃) can be used for both the first channel 305as well as the source/drain regions 401 such that there is no latticemismatch between the first channel 305 and the source/drain regions 401.As such, the overall process may be simplified while taking advantage ofthe properties of the topological insulators in the formation oftransistors. For example, by forming the single-gate transistor 400 withthe first channel 305 and the source/drain regions 401 as describedabove, a normally-off topological insulator based transistor with a lowcontact resistance may be obtained. Additionally, because materials suchas Bi₂Se₃ have a layered crystal structure consisting of stackedSe—Bi—Se—Bi—Se quintuple layers (QLs), and since the channel thicknessin some embodiments is nearly 1 QL, the short channel effects in devicesthat utilized these embodiments is the same as FETs based ontwo-dimensional materials. In other words, short-channel effects can besuppressed by reducing the gate dielectric thickness, which enhances theelectrostatic control from the gate.

FIGS. 5A-5C illustrate an embodiment of a process that may be used tomanufacture a second single-gate transistor 500 using, e.g., atopological insulator. In this embodiment the first channel layer 103 isformed on the substrate 101 as described above with respect to FIG. 1(e.g., an epitaxial growth process). In this embodiment, however, thefirst channel layer 103 is initially grown to the second thickness T₂which is larger than the critical thickness T_(c) of the material chosenfor the first channel layer 103. As such, the first channel layer 103 inthis embodiment is initially formed with the properties of thetopological insulator, with both the bulk insulator along with theconductive surface.

FIG. 5B illustrates that, once the first channel layer 103 has beengrown to the second thickness T₂, the first channel 305 and thesource/drain regions 401 are formed from the first channel layer 103(with the separation between the first channel layer 103 and thesource/drain regions 401 being illustrated by the dashed lines in FIG.5B). In an embodiment the first channel 305 and the source/drain regions401 are formed simultaneously from the first channel layer 103 (with thesecond thickness T₂) by patterning the first channel layer 103 andreducing the thickness of the first channel layer 103 until the firstchannel 305 has the first thickness T₁ below the critical thicknessT_(c) such that the first channel 305 has the properties of asemiconductor material.

The thickness of the first channel layer 103 may be reduced by initiallyplacing a source/drain mask (not separately illustrated in FIG. 5B) overthose portions of the first channel layer 103 that are desired to beformed into the source/drain regions 401. In an embodiment thesource/drain mask may be deposited and patterned using, for example,deposition and photolithography techniques known in the art. Thesource/drain mask may incorporate commonly used masking materials, suchas (but not limited to) photoresist material, silicon oxide, siliconoxynitride, and/or silicon nitride. Once the source/drain mask has beenplaced, the exposed portions of the first channel layer 103 may beetched using plasma etching to form the first channel 305 (with thethickness reduced to below the critical thickness T_(c)) and thesource/drain regions 401 (without the thickness reduced to below thecritical thickness T_(a)).

FIG. 5C illustrates that, once the first channel 305 and thesource/drain regions 401 have been formed from the first channel layer103, the first gate dielectric 303 and the first gate electrode 301 maybe formed over the first channel 305. In an embodiment the first gatedielectric 303 may be formed as described above with respect to FIGS. 1and 3 . For example, the first gate dielectric layer 105 (not separatelyillustrated in FIG. 5C) may be formed by initially forming a layer ofmaterial such as aluminum oxide or hafnium oxide using a process such asatomic layer deposition, chemical vapor deposition, sputtering, or thelike. By using a conformal deposition process such as atomic layerdeposition, the first gate dielectric layer 105 will take on the shapeof the underlying structures, forming a “U” shape between thesource/drain regions 401 and over the first channel 205.

Once the first gate dielectric layer 105 has been deposited, the firstgate electrode layer 107 (not separately illustrated in FIG. 5C) may bedeposited over the first gate dielectric layer 105. In an embodiment thefirst gate electrode layer 107 may be formed as described above withrespect to FIG. 1 . For example, the first gate electrode layer 107 maybe deposited by depositing a conductive material such as gold, titanium,platinum, or aluminum over the dielectric material using a process suchas sputtering, although any other suitable material or method ofmanufacture may be utilized.

Once the first gate dielectric layer 105 and the first gate electrodelayer 107 have been formed, the first gate dielectric layer 105 and thefirst gate electrode layer 107 may be patterned into the first gatedielectric 303 and the first gate electrode 301, respectively. In anembodiment the first gate dielectric layer 105 and the first gateelectrode layer 107 may be patterned as described above with respect toFIG. 3 , whereby one or more photoresists are deposited, exposed,developed, and then used as mask(s) in order to pattern the first gatedielectric layer 105 and the first gate electrode layer 107 into thedesired shapes. In an embodiment the first gate electrode 301 may bepatterned to a fourth width W₄ of between about 7 nm and about 100 μm,and the first gate dielectric 303 may be patterned to a fifth width W₅of between about 7 nm and about 100 μm.

FIG. 6A illustrates another embodiment in which the first channel 305and the source/drain regions 401 are formed over the first gateelectrode 301 and the first gate dielectric 303. In this embodiment thefirst gate electrode 301 and the first gate dielectric 303 are initiallyformed prior to the formation of the first channel 305 and thesource/drain regions 401. In an embodiment the first gate electrode 301and the first gate dielectric 303 may be formed as described above withrespect to FIGS. 1 and 3 . For example, the first gate dielectric layer105 and the first gate electrode layer 107 may be formed by initiallyforming a conductive material such as gold, titanium, platinum, oraluminum over the dielectric material using a process such as sputteringand then depositing a layer of material such as aluminum oxide, hafniumoxide, or silicon oxide using a process such as atomic layer deposition.Once the first gate dielectric layer 105 and the first gate electrodelayer 107 have been formed, the first gate dielectric layer 105 and thefirst gate electrode layer 107 may be patterned, if desired, using asuitable photolithographic masking and etching process in order to formthe first gate electrode 301 and the first gate dielectric 303. However,any suitable process for depositing the first gate electrode 301 and thefirst gate dielectric 303 may be utilized. However, in this embodimentthe first gate dielectric layer 105, if desired, may be formed to athickness of between about 50 nm to about 500 nm, although any suitablethickness may be utilized.

Once the first gate electrode 301 and the first gate dielectric 303 havebeen formed, the first channel 305 and the source/drain regions 401 maybe formed over the first gate dielectric 303. In an embodiment the firstchannel 305 and the source/drain regions 401 may be formed as describedabove with respect to FIGS. 1-5B. For example, the first channel 205 andthe source/drain regions 401 may be formed by initially growing thefirst channel layer 103 to the second thickness T₂ such that thematerial of the first channel layer 103 has the properties of atopological insulator (with metallic conducting surface states), andthen reducing the thickness of a portion of the original first channellayer 103 to the first thickness T₁ in order to form the first channel305 with properties of a semiconductor (as described above with respectto FIGS. 5A-5B). As such, the first gate dielectric 303 has a planarsurface facing the first channel 305 and the source/drain region 401 isin physical contact that planar surface.

In another embodiment similar to the embodiment described above withrespect to FIGS. 1-4B, the first channel layer 103 may be grown on thefirst gate dielectric 303 to the first thickness T₁ and without growingto a thickness greater than the critical thickness T_(c). As such, thefirst channel layer 103 is formed to have properties of a semiconductormaterial. Once the first channel layer 103 has been grown to the firstthickness T₁, the first channel layer 103 may be patterned into thefirst channel 305 by placing, exposing, and developing a photoresist(not separately illustrated in FIG. 6A) over the first channel layer 103before using the photoresist as a mask during an etching process such asa dry etch to form the first channel 305.

In addition to forming the first channel 305, the etching process willalso expose the underlying first gate dielectric 303. Once theunderlying first gate dielectric 303 has been exposed, the source/drainregions 401 may be formed on opposite sides of the first channel 305 asdescribed above with respect to FIG. 4A. For example, the source/drainregions 401 may be formed from the same material as the first channel305 using an epitaxial growth process to epitaxially grow thesource/drain regions 401 onto the first gate dielectric 303.Additionally, the source/drain regions 401, while being grown from thesame material as the first channel 305, will grow the source/drainregions 401 to have the second thickness T₂ greater than the criticalthickness T_(c) such that the source/drain regions 401 will have theproperties of a topological insulator with metallic surface states. Forexample, in an embodiment in which the source/drain regions 401 areBi₂Se₃, the source/drain regions 401 will be grown to a thickness thatis greater than six quintuple layers. However, any suitable thicknessmay be utilized. As such, the first gate dielectric 303 in thisembodiment has a first planar surface facing the first channel 305 and asecond planar surface perpendicular to the first planar surface, whereinthe source/drain region 401 is in physical contact the second planarsurface.

FIG. 6B illustrates a top-down view of the structure of FIG. 6A. In thisview, it can be seen that the first channel 305 is over the first gatedielectric 303 and extends between the source/drain regions 401. In anembodiment the first channel 305 in this structure has a fourth lengthL₄ of between about 7 nm and about 100 μm, while the source/drainregions 401 have a fifth length L₅ of between about 7 nm and about 100μm. However, any suitable dimensions may be utilized.

By forming the first channel 305 and the source/drain regions 401 overthe first gate dielectric 303, additional flexibility in themanufacturing process may be obtained. Such flexibility allowsmanufacturers the ability to modify their processes to arrive at themost efficient use of resources, allowing for a more efficient process.

FIGS. 7-8B illustrate another embodiment in which a multiple channel,multiple gate transistor 800 is formed. In this embodiment, the firstchannel layer 103 is grown over the first gate dielectric 303, which isalready formed and located over the first gate electrode 301. Forexample, the first channel layer 103 is formed to the first thickness T₁(less than the critical thickness T_(c)) such that the first channellayer 103 has the properties of a semiconductor material, such as bybeing less than 6 quintuple layers in an embodiment in which the firstchannel layer 103 is Bi₂Se₃.

Once the first channel layer 103 has been formed, a first dielectriclayer 701 may be formed over the first channel layer 103 in order toseparate and isolate the first channel layer 103 (which will become thefirst channel 305 in the multiple channel, multiple gate transistor 800)from a second channel layer 703 (which will become a second channel 805in the multiple channel, multiple gate transistor 800). In an embodimentthe first dielectric layer 701 may be dielectric material such asaluminum oxide or hafnium oxide that is formed using a process such asALD, CVD, PVD, combinations of these, or the like. The first dielectriclayer 701 may be formed to have an equivalent oxide thickness of betweenabout 0.5 nm and about 2 nm, although any suitable thickness may beutilized.

Once the first dielectric layer 701 has been formed, a second channellayer 703 may be formed on the first dielectric layer 701. In anembodiment the second channel layer 703 will be used to form the secondchannel 805 of the multiple channel, multiple gate transistor 800 and,as such, may be similar to the first channel layer 103. For example, thesecond channel layer 703 may be formed from a topological insulatormaterial such as Bi₂Se₃ and may be formed to have the first thickness T₁which is below the critical thickness T_(c) of the material used for thesecond channel layer 703. As such, the second channel layer 703 willhave the properties of a semiconductor material. In a particularembodiment in which Bi₂Se₃ is used as the material for the secondchannel layer 703, the second channel layer 703 may be formed to havethe first thickness T₁ of less than about six quintuple layers, althoughany suitable thickness may be utilized.

After the second channel layer 703 has been formed, a second gatedielectric layer 705 is formed over the second channel layer 703. In anembodiment the second gate dielectric layer 705 may be formed fromsimilar materials and using similar processes as the first gatedielectric layer 105 described above with respect to FIG. 1 . Forexample, the second gate dielectric layer 705 may be formed from adielectric material such as aluminum oxide or hafnium oxide to athickness of between about 0.5 nm and about 2 nm using a process such asALD, CVD, PVD, or the like. However, any suitable material or method ofmanufacturing may be utilized.

Once the second gate dielectric layer 705 has been formed, a second gateelectrode layer 707 may be formed over the second gate dielectric layer705. In an embodiment the second gate electrode layer 707 may be formedfrom similar materials and using similar processes as the first gateelectrode layer 107 described above with respect to FIG. 1 . Forexample, the second gate electrode layer 707 may be formed from aconductive material such as gold, titanium, platinum, aluminum, or thelike using a process such as ALD, CVD, PVD, or the like. However, anysuitable material or method of manufacturing may be utilized.

FIG. 8A illustrates a formation of the source/drain regions 401. In anembodiment the formation of the source/drain regions 401 may beinitiated by first patterning the second gate electrode layer 707 into asecond gate electrode 801. In an embodiment the second gate electrodelayer 707 is patterned in a similar fashion as the first gate electrodelayer 107 was patterned into the first gate electrode 301 (describedabove with respect to FIG. 3 ). For example, a photoresist may beplaced, exposed, developed, and used as a mask in a photolithographicmasking and etching process in order to pattern the second gateelectrode layer 707 into the second gate electrode 801. In an embodimentthe second gate electrode 801 may be formed to have a sixth width W₆ ofbetween about 7 nm and about 100 μm.

Once the second gate electrode 801 has been formed, the second gatedielectric layer 705, the second channel layer 703, the first dielectriclayer 701, and the first channel layer 103 may next be patterned for theeventual formation of the source/drain regions 401. In an embodiment thesecond gate dielectric layer 705, the second channel layer 703, firstdielectric layer 701, and the first channel layer 103 may be patternedby initially placing, exposing, and developing a photoresist over thesecond gate electrode 801 and exposed second gate dielectric layer 705and then using the photoresist as a mask during an etching process suchas a dry etching process in order to etch through the second gatedielectric layer 705, the second channel layer 703, the first dielectriclayer 701, and the first channel layer 103 until the first gatedielectric 303 has been exposed. The patterning will form a second gatedielectric 803 (from the second gate dielectric layer 705), a secondchannel 805 (from the second channel layer 703) and the first channel305 (from the first channel layer 103). In an embodiment the second gatedielectric 803, the second channel 805 and the first channel 305 may beformed to have a seventh width W₇ of between about 7 nm and about 100μm. although any suitable dimension may be utilized.

Once the underlying first gate dielectric 303 has been exposed, thesource/drain regions 401 may be formed on opposite sides of the firstchannel 305 and the second channel 805, and the source/drain regions 401may be formed as described above with respect to FIG. 4A. For example,the source/drain regions 401 may be formed from the same material as thefirst channel 305 and the second channel 805 and using an epitaxialgrowth process to epitaxially grow the source/drain regions 401 onto theexposed first gate dielectric 303. Additionally, the source/drainregions 401, while being grown from the same material as the firstchannel 305 and the second channel 805, will grow the source/drainregions 401 to have the second thickness T₂ that is at least greaterthan the critical thickness T_(c) of the material used for thesource/drain regions 401 such that the source/drain regions 401 willhave the properties of a topological insulator with metallic surfacestates. For example, in an embodiment in which Bi₂Se₃ is used as thematerial for the source/drain regions 401, the source/drain regions 401may be grown to the second thickness T₂ that is greater than sixquintuple layers. However, any suitable thickness may be utilized.

FIG. 8B illustrates a top down view of the multiple channel, multiplegate transistor 800. In this embodiment the second gate dielectric 803may be formed to have a sixth length L₆ of between about 7 nm and about100 μm. while the second gate electrode 801 may be formed to have aseventh length L₇ of between about 7 nm and about 100 μm. Additionally,the source/drain regions 401 may be formed to have an eighth length L₈of between about 7 nm and about 100 μm. However, any suitable dimensionsmay be utilized.

By utilizing the processes described above with respect to FIGS. 7-8B, amultiple channel, multiple gate transistor 800 may be formed. Such atransistor allows for a normally off transistor with the use oftopological materials which also has the benefits that multiple channelsand multiple gates provide.

In accordance with an embodiment, a method of manufacturing asemiconductor device comprising growing a layer of a first material to afirst thickness onto a substrate, the first thickness being less than acritical thickness is provided. A gate dielectric layer and a gateelectrode layer are deposited over the layer of the first material andthe gate dielectric layer and the gate electrode layer are patternedinto a gate stack. The first layer of material is patterned to expose aportion of the substrate, and source/drain regions are grown onto theportion of the substrate, wherein the growing the source/drain regionsgrows the first material to a thickness greater than the criticalthickness.

In accordance with another embodiment, a method of manufacturing asemiconductor device comprising growing a layer of a first material to afirst thickness onto a substrate, the first thickness being greater thana critical thickness, is provided. A portion of the first material isremoved to form a channel region and an opening over the channel region,wherein the removing the portion of the first material reduces thethickness of at least a portion of the first material to less than thecritical thickness and also modifies the properties of the firstmaterial within the channel region. A gate dielectric is formed withinthe opening, and a gate electrode is formed over the gate dielectric.

In accordance with yet another embodiment, a semiconductor devicecomprising a first channel region comprising a first material, whereinthe first material has a critical thickness below which the firstmaterial has properties of a semiconductor material and above which thefirst material has properties of a topological insulator, wherein thefirst channel region has a first thickness less than the criticalthickness is provided. A source/drain region is adjacent to the firstchannel region, wherein the source/drain region comprises the firstmaterial with a second thickness greater than the critical thickness. Agate dielectric is adjacent to the first channel region, and a gateelectrode is on an opposite side of the gate dielectric from the firstchannel region.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor device comprising: a firstmaterial on a top surface of a substrate with a channel region and asource/drain region, the channel region having a first thickness lessthan a critical thickness below which the first material has propertiesof a semiconductor material and above which the first material hasproperties of a topological insulator, the source/drain region beingadjacent to the channel region and having a second thickness greaterthan the critical thickness, wherein the first thickness and the secondthickness are defined along a direction perpendicular to the top surfaceof the substrate; a gate dielectric adjacent to the channel region; anda gate electrode adjacent to the gate dielectric.
 2. The semiconductordevice of claim 1, wherein the first material is Bi₂Te₃.
 3. Thesemiconductor device of claim 1, wherein the first material is Sb₂Te₃.4. The semiconductor device of claim 1, wherein the first material isBi₂Te₂Se.
 5. The semiconductor device of claim 4, wherein the firstmaterial is Bi₂Te₂S.
 6. The semiconductor device of claim 1, wherein thefirst material is Bi₂Se₂S.
 7. The semiconductor device of claim 1,wherein the first material is Sb₂Te₂Se.
 8. A semiconductor devicecomprising: a first material with a critical thickness below which thefirst material has properties of a semiconductor material and abovewhich the first material has properties of a topological insulator; agate dielectric extending between and directly contacting a firstthickness of the first material and a second thickness of the firstmaterial, the first thickness less than the critical thickness and thesecond thickness greater than the critical thickness; and a gateelectrode adjacent to the gate dielectric.
 9. The semiconductor deviceof claim 8, wherein the first thickness is less than six quintuplelayers.
 10. The semiconductor device of claim 8, further comprising asecond material with a second critical thickness below which the secondmaterial has properties of a second semiconductor material and abovewhich the second material has properties of a second topologicalinsulator, the second material being located over the first material.11. The semiconductor device of claim 8, wherein the first material isSb₂Te₂Se.
 12. The semiconductor device of claim 8, wherein the firstmaterial is Bi₂Se₂S.
 13. The semiconductor device of claim 8, whereinthe first material is Bi₂Te₂S.
 14. The semiconductor device of claim 8,wherein the first material is Bi₂Te₂Se.
 15. A semiconductor devicecomprising: a first semiconductor region extending between a firsttopological insulator and a second topological insulator, wherein thefirst semiconductor region, the first topological insulator, and thesecond topological insulator are the same material, each with adifferent thickness; a gate dielectric overlying the first semiconductorregion, wherein a portion of the gate dielectric extends between anddirectly contacts the first topological insulator and the secondtopological insulator; and a gate electrode overlying the gatedielectric.
 16. The semiconductor device of claim 15, wherein firstsemiconductor region is an epitaxial material.
 17. The semiconductordevice of claim 15, wherein the same material is Bi₂Se₃.
 18. Thesemiconductor device of claim 15, wherein the first semiconductor regionis less than six quintuple layers.
 19. The semiconductor device of claim15, wherein the first semiconductor region is part of a multiple channeltransistor.
 20. The semiconductor device of claim 15, further comprisinga second semiconductor region extending between the first topologicalinsulator and the second topological insulator, the second semiconductorregion being separated from the first semiconductor region by adielectric material.